Pulse-width modulated motor control systems for AC motors are well known, as exemplified by U.S. Pat. No. 4,052,648, assigned to the U.S. Government, which describes a power factor control for single phase motors over an extended load range of the motor. This concept is widely known and used on many single phase induction motors because it's an easy and inexpensive means of accomplishing voltage control. However, to accomplish this voltage control with a polyphase induction motor becomes much more complicated because of inherent instability problems and leads to a more expensive and less efficient system than the single phase version.
In the area of polyphase AC motors, the introduction of the high voltage Darlington power transistor configuration has lead to the development of three phase inverters whose capabilities provide a variable output as a function of voltage and frequency, which has a linear relationship. As used herein, the term "linear" refers to operating on a linear curve, which is determined by the rated voltage of the motor at rated frequency. Such a curve is exemplified at W in FIG. 1.
The present three phase inverter designs do not easily allow access to the measurement of the power factor angle of the motor, and subsequent regulation of the system to maintain a substantially constant power factor or apparent power factor to maximize economy of operation of the motor.